The present invention relates to thermal dye diffusion printing.
Thermal dye diffusion printing is a non-impact electronic printing process which is capable of giving near photographic quality prints. An image is formed by using a thermal head to transfer dye from a color ribbon into a surface of a receiver. The head consists of a line of individually addressable heated elements which are fed appropriate electrical pulses to generate the quantity of heat required to provide the amount of dye transfer needed to reproduce the corresponding pixel in the image. The surface temperature of the thermal head typically is 350.degree. C. and the duration of the heating pulse typically is up to 10 milliseconds. The depth of shade is governed by the length of the heating pulse and a full color image is built up in the normal way by transfer of the primary colors on top of each other.
In general, both the ribbon and the receiver have very smooth surfaces. The active layer of the ribbon consists of a solid solution of dye in a binder. The receiver typically consists of a polymer coating on a paper or other supporting material; the coating is designed to be receptive to the dye and to release smoothly from the ribbon, or dye layer, after printing.
The receiver is supported on a platen roller and the thermal head is pressed against the surface of the ribbon. The two media are held together under a pressure of between 10 and 100 atmospheres. As the surfaces are very smooth, there is no air gap between the media in the region where the thermal head is applying maximum pressure. Dye is transferred by phase transfer and diffusion; the dye which is dissolved in the dye layer partitions into the receiver coating by a molecular diffusion process. Once dye is in the receiver coating, it is free to continue diffusing further into the coating as long as the thermal head temperature remains sufficiently high. At all stages, the dye molecules are surrounded by a polymer matrix; transfer occurs from one polymer phase to another without sublimation taking place.
Because the dyes are always constrained by polymer molecules, the dye diffusion process is very well controlled. The build up of color is well defined and sideways diffusion cannot occur, as is possible in the air gap of a sublimation process.
While films have the requisite degree of smoothness, the accumulation of a static charge on the film may occur during the printing process which leads to the attraction of dust particles on the surfaces of the film. The presence of dust on a receiver coating prevents intimate contact over the entire area of the thermal printing head which results in imperfections in the printed image. While papers generally do not suffer from static charge build up, image quality may be reduced. In addition, transfer of the printed image to another substrate may be adversely affected by the use of a paper carrier or support for the receiver coating. Accordingly, there is a need for improved thermal dye diffusion substrates, particularly for substrates intended to be used as a heat transfer material.